1. Field of the Invention
The present invention relates to a semiconductor manufacturing technique. More particularly, the present invention relates to a method of enhancing adhesion strength of a boro-silicate glass (BSG) film to a silicon nitride film. The method according to the present invention can effectively prevent peeling of the BSG film from the silicon nitride film during or after a dry etching process, and is capable of integrating with standard semiconductor manufacturing processes.
2. Description of the Prior Art
During the manufacturing of semiconductor devices, a number of film deposition and etching processes are involved. As known by those skilled in the art, adhesion property between films in a semiconductor wafer is one of the most important factors that affects yield of these devices. Please refer to FIG. 1 to FIG. 5. FIG. 1 to FIG. 5 are schematic diagrams depicting a prior art method of forming a deep trench on a semiconductor substrate 10. As shown in FIG. 1, a pad oxide layer 12 and a silicon nitride film 14 are provided on the semiconductor substrate 10. Typically, the silicon nitride film 14 is formed by using low-pressure chemical vapor deposition (LPCVD). The silicon nitride film 14 has a thickness of about 1000 to 2000 angstroms.
As shown in FIG. 2, a thick boro-silicate glass (BSG) film 16 is then deposited onto the silicon nitride film 14. The BSG film 16 is used as a hard mask film in the succeeding dry etching processes. Typically, the BSG film 16 has a thickness of about 7,500 angstroms to 15,000 angstroms and a boron concentration of about 5.5% in weight.
As shown in FIG. 3, a patterned photoresist layer 18 is coated on the BSG film 16. The photoresist layer 18 has openings 19 that define the location and pattern of deep trenches to be formed in the semiconductor substrate 10. As shown in FIG. 4, a first dry etching process is carried out to etch the BSG film 16 and the silicon nitride film 14 through the openings 19, thereby forming openings 21 in the films 16 and 14. Subsequently, as shown in FIG. 5, a second dry etching process is performed to etch the pad oxide layer 12 and the semiconductor substrate 10 by using the BSG film 16 as a hard mask film, thereby forming deep trenches 23.
However, as indicated by numeral 25 in FIG. 5, edge peeling is frequently observed in the interface between the BSG film 16 and the silicon nitride film 14 around the deep trenches 23. The peeling problems, which might lead to costly failure of deep trench pattern transfer, usually occur during the dry etching processes. One approach to solving the problem of peeling between the BSG film 16 and the underlying silicon nitride film 14 is to reduce the boron concentration of the BSG film 16 down to a value of below 5% in weight. Unfortunately, such a reduced boron concentration in the BSG film 16 will cause difficulty in removing the BSG film 16 in the follow-up cleaning procedures. As known by those skilled in the art, the higher the boron concentration in the BSG film 16 is, the easier the BSG film 16 can be removed in a wet cleaning procedure that employs a typical etching solution such as H2SO4/HNO3 mixture.
Consequently, there is a strong need to provide a method that is capable of maintaining the boron concentration in the BSG film 16 at an acceptable level, while enhancing the adhesion strength of the BSG film 16 to the silicon nitride film 14. Further, according to the prior art deep trench process, no surface treatment is performed on the silicon nitride film 14 before the deposition of the BSG film 16.